## How to calculate the configuration of special solar battery in off-grid photovoltaic power generation system

Off-grid photovoltaic power generation system is composed of solar cell components, controller, inverter, battery pack and bracket system. They can generate DC power directly during the day or stored in a special solar battery pack to provide electricity at night or on cloudy or rainy days. In the process of designing off-grid photovoltaic power generation systems, an indispensable part is the design of batteries. Including the capacity design of the battery and the series-parallel design of the battery pack. The manufacturer of special solar batteries will introduce to you how to calculate the configuration of special solar batteries in off-grid photovoltaic power generation systems.

One, the basic capacity of the battery and the design of series and parallel connections

The design of the battery includes the design of the battery capacity and the series-parallel design of the battery pack. 1. Basic formula 1. The basic method of calculating battery capacity. Step I: Multiply the daily power consumption required by the load by the number of self-sufficient days determined according to the actual situation to get the preliminary battery capacity. II. In the second step, divide the battery capacity obtained in the step by the allowable maximum discharge depth of the battery. Because the battery cannot be fully discharged in self-sufficient days, it is necessary to divide by the large depth of discharge to get the required battery capacity. The selection of the maximum depth of discharge requires reference to the performance parameters of the battery selected for use in the photovoltaic system, and detailed information about the maximum depth of discharge of the battery can be obtained from the battery supplier. Under normal circumstances, if you are using a deep cycle battery, it is recommended to use 80% depth of discharge (DOD); if you are using a shallow cycle battery, it is recommended to use 50% (DOD). The basic formula for designing battery capacity is as follows: Self-sufficient days X daily average load/maximum depth of discharge = battery capacity 2. Determine the basic method of battery series and parallel connection. Each battery has its nominal voltage. In order to reach the nominal voltage of the load, we connect the batteries in series to supply power to the load. The number of batteries that need to be connected in series is equal to the nominal voltage of the load divided by the nominal voltage of the battery. Load nominal voltage / battery nominal voltage = number of batteries in series

In order to illustrate the application of the above basic formula, we use a small AC photovoltaic application system as an example. Assuming that the power consumption of the AC load of the photovoltaic system is 10KWh/day, if the efficiency of the inverter we choose to use in the photovoltaic system is 90% and the input voltage is 24V, then the required DC load demand is 462.96Ah/day. (10000 Wh ÷ 0.9 ÷ 24 V = 462.96 Ah). We assume that this is a system where the load is not very strict on the power requirements, and the user can adjust the power consumption according to the weather conditions more flexibly. We choose 5 days of self-sufficiency and use deep-cycle batteries with a discharge depth of 80%. Then: Battery capacity=5 days×462.96Ah/0.8=2893.51Ah. If you choose 2V/400Ah single battery, then the number of batteries that need to be connected in series = 24V/2V = 12 (pcs) The number of batteries that need to be connected in parallel = 2893.51/400 = 7.23 We take the integer 8. Therefore, the number of 2V/400Ah batteries that the system needs to use is: 12 series × 8 parallel = 96 (pcs).

The following is an example of a pure DC system: the photovoltaic power supply system of a country house. The cabin is only used on weekends, and low-cost shallow-cycle batteries can be used to reduce system costs. The load of this country house is 90 Ah/day, and the system voltage is 24V. We choose the number of self-sufficiency days as 2 days, and the allowable maximum discharge depth of the battery is 50%, then: battery capacity = 2 days × 90Ah/0.5 = 360Ah. If you choose a 12V/100Ah battery, then you need the battery 2 in series × 4 in parallel = 8 batteries. 2. Design revision

The above is only the basic estimation method of battery capacity. In actual situations, there are many performance parameters that will have a great impact on battery capacity and service life. In order to get the correct battery capacity design, the basic equation above must be revised. For batteries, the capacity of a battery is not static. The capacity of a battery is related to two important factors: the discharge rate of the battery and the ambient temperature.

1. First of all, we consider the impact of discharge rate on battery capacity. The capacity of the battery changes with the change of the discharge rate. As the discharge rate decreases, the capacity of the battery will increase accordingly. This will have an impact on our capacity design. When designing a photovoltaic system, it is necessary to select the battery capacity at an appropriate discharge rate for the designed system. Usually, the manufacturer provides the battery capacity at the rated capacity of the battery at a discharge rate of 10 hours. But in the photovoltaic system, because the energy stored in the battery is mainly used for the load in self-sufficient days, the battery discharge rate is usually slow. The typical discharge rate of the battery in the photovoltaic power supply system is 100 to 200 hours. When designing, we need to use the concept of average discharge rate commonly used in battery technology. The formula for the average discharge rate of the photovoltaic system is as follows:

number of days of self-sufficiency X load working time / maximum discharge depth = average discharge rate (hours)

The load working time in the above formula can be estimated by the following method: For a photovoltaic system with only a single load, the working time of the load is the average working hours of the actual load per day; for a photovoltaic system with multiple different loads, the working time of the load The weighted average load working time can be used, and the actual average discharge rate of the photovoltaic system can be calculated according to the above two formulas. According to the battery capacity of this type of battery provided by the battery manufacturer at different discharge rates, the battery capacity can be corrected . 2. Consider the influence of temperature on battery capacity below. The capacity of the battery will change as the temperature of the battery changes. When the temperature of the battery drops, the capacity of the battery will decrease. Generally, the capacity of lead-acid batteries is calibrated at 25°C. As the temperature decreases, the capacity at 0°C drops to about 90% of the rated capacity, and at -20°C it drops to about 80% of the rated capacity. Therefore, the influence of the battery's ambient temperature on its capacity must be considered.

If the temperature of the photovoltaic system installation site is very low, it means that the actual use capacity of the battery capacity designed according to the rated capacity in the area will be reduced, that is, it cannot meet the power demand of the system load. In actual work, it will lead to over-discharge of the battery, reduce the service life of the battery, and increase the maintenance cost. In this way, the battery capacity required in the design is larger than the capacity calculated according to the battery parameters under the standard condition (25°C). Only by choosing and installing a capacity with more than the calculated capacity at 25°C can the battery be ensured that the temperature is lower than At 25°C, it can fully provide the required energy.

Battery manufacturers generally provide related battery temperature-capacity correction curves. The battery capacity correction coefficient corresponding to the temperature can be found on the curve, and the preliminary calculation result of the battery capacity can be corrected by dividing by the battery capacity correction coefficient. The above is a typical temperature-discharge rate-capacity change curve. Because of the influence of low temperature, a factor that must be considered in the battery capacity design is to modify the large depth of discharge of the battery to prevent the battery from freezing and failing at low temperatures, causing damage to the battery. The electrolyte in the lead-acid battery is at low temperature

may solidify, as the battery discharges, the water continuously generated in the battery dilutes the electrolyte, causing the condensation point of the battery electrolyte to continue to rise until the pure water is 0°C. In cold weather conditions, if the battery discharges too much, as the condensation point of the electrolyte rises, the electrolyte may condense and damage the battery. Even if a deep-cycle industrial battery is used in the system, its maximum depth of discharge should not exceed 80%. The following figure shows the relationship between the maximum discharge depth of a general lead-acid battery and the battery temperature. You can refer to this figure to obtain the required adjustment factor during system design.

When designing, use the low average temperature of the area where the photovoltaic system is located, and then find the maximum allowable discharge depth of the battery used in the area from the above chart or the maximum discharge depth-battery temperature relationship diagram provided by the battery manufacturer. Normally, correction is only considered when the temperature is below minus 8 degrees. 3. Complete design calculation of battery capacity Considering all the above calculation correction factors, we can get the following final calculation formula of battery capacity.

Self-sufficient days X average daily load battery capacity (@specified discharge rate)=--------------------------------- Maximum allowable Discharge depth X temperature correction factor The following summarizes and analyzes each parameter: 1. Maximum allowable depth of discharge: Generally speaking, the maximum allowable depth of discharge of shallow-cycle batteries is 50%, and the maximum allowable depth of discharge of deep-cycle batteries is 80 %. If you are in a severe cold area, you must take into account the low temperature antifreeze problem and make the necessary corrections. When designing, this value can be appropriately reduced to expand the capacity of the battery to extend the service life of the battery. For example, if a deep-cycle battery is used, when designing, set the maximum available percentage of the battery capacity to 60% instead of 80%. This can increase the service life of the battery and reduce the maintenance cost of the battery system. The cost will not have much impact. This can be handled flexibly according to the actual situation. 2. Temperature correction factor: When the temperature decreases, the capacity of the battery will decrease. The function of the temperature correction coefficient is to ensure that the installed battery capacity is greater than the capacity value calculated according to the 25°C standard, so that the designed battery capacity can meet the power demand of the actual load. 3. Designated discharge rate: The designated discharge rate takes into account that the slower discharge rate will get more capacity from the battery. Using the data provided by the supplier, you can select the appropriate battery capacity at the specified discharge rate suitable for the design of the system. If there is no detailed information about the capacity-discharge rate, it can be roughly estimated that the

In the case of 　　 discharge rate (C/100 to C/300), the capacity of the battery is 30% more than the standard state. 4. The following examples illustrate the application of the above formula. Establish a photovoltaic power supply system to supply power to a remote communication base station. The system has two loads: load one, the working current is 1 ampere, and it works 24 hours a day. Load two, working current is 5 amps and work 12 hours a day. The 24-hour average low temperature of the location where the system is located is -20°C, and the self-sufficiency time of the system is 5 days. Use deep-cycle industrial batteries (large DOD is 80%). Because the 24-hour average low temperature in the area where the photovoltaic system is located is -20°C, the maximum allowable discharge depth of the battery must be corrected. We can determine the maximum allowable depth of discharge as 50% from the graph of the relationship between the maximum depth of discharge and the battery temperature. Therefore: Weighted average load working time = = 6.67hrs Average discharge rate = = 66.7 hours rate According to the typical temperature-discharge rate-capacity curve on the previous page, the calculated value of the average discharge rate is close to the 50-hour rate. The temperature correction coefficient corresponding to the discharge rate at -20°C is 0.7 (you can also query according to the performance table provided by the supplier). If the calculated discharge rate is between the two data, the faster discharge rate (short time) is more conservative and reliable. So the battery capacity is: battery capacity =

1428.57Ah@50 hour discharge rate According to the battery parameter table provided by the supplier, we can select the appropriate battery to connect in series and parallel to form the required battery pack.

Fourth, the battery pack parallel design After calculating the required battery capacity, the next step is to decide how many single batteries to choose in parallel to obtain the required battery capacity. There are many options. For example, if the calculated battery capacity is 500Ah, then we can choose a single 500Ah battery, or two 250Ah batteries in parallel, or five 100Ah batteries in parallel. In theory, these options can meet the requirements, but in practical applications, the number of parallel connections should be minimized. That is to say, it is better to choose a large-capacity battery to reduce the number of parallel connections required. The purpose of this is to minimize the impact caused by the imbalance between the batteries, because some parallel batteries may be unbalanced with the parallel batteries during charging and discharging. The more groups connected in parallel, the greater the possibility of battery imbalance. Generally speaking, it is recommended that the number of parallel connections should not exceed 4 groups. At present, many photovoltaic systems use two parallel modes. In this way, if a set of batteries fails and cannot work normally, you can disconnect the set of batteries for maintenance, and use another set of normal batteries. Although the current has dropped, the system can maintain normal operation at the nominal voltage. . In short, the parallel design of battery packs needs to consider different actual conditions and make different choices according to different needs.